JP-2008-196318A shows a spark ignition internal combustion engine in which gasoline containing alcohol, such as ethanol, is combusted. Especially, JP-2008-196318A shows a technology in which a stratified combustion is stably conducted.
As the quantity of alcohol contained in gasoline increases, a fuel injection time period is more prolonged, whereby the stratified combustion is deteriorated. An engine control system shown in JP-2008-196318A is provided with an alcohol sensor detecting alcohol concentration in fuel. When the alcohol concentration is increased, a spray-guided fuel injection is switched into a wall-guided fuel injection so that the stratified combustion is well conducted.
However, in this engine control system, the viscosity of the fuel is not considered, although the viscosity of the fuel is not constant.
(i) For example, the viscosity of gasoline depends on the temperature thereof. As the temperature is decreased, the viscosity increases. That is, even if the fuel is gasoline including no alcohol, the viscosity of the fuel increases in cold climates.
(ii) In a case the fuel is gasoline including alcohol, since the viscosity of alcohol is greater than that of gasoline, the viscosity of the fuel increases along with an increase in contained alcohol.
When the viscosity of the fuel is varied, the shape of fuel spray injected from a fuel injector is also varied. Specifically, when the viscosity of the fuel is relatively small, for example, the viscosity of gasoline at ordinary temperature, the fuel spray injected from a fuel injector is spread as shown by a long dashed short dashed line α in FIG. 4A. As the viscosity of the fuel increases, the fuel spray is narrowed as shown by a solid line β in FIG. 4A.
As above, the shape of the fuel spray is varied according to a variation in fuel viscosity and the distance between a discharge portion of a spark plug and the fuel spray is also varied. However, in a conventional engine control system, since the viscosity of the fuel is not considered, the fuel combustion condition may be deteriorated due to a variation in fuel viscosity.